Complex Phase Behavior in Thermoelectric Mg2Si1-xSnx Alloys and Development of Transient Plane Source Method for Structural Characterization
Doctoral thesis, 2023
The thermoelectric effect was discovered nearly 200 years ago, but it started to be utilized for waste heat recovery only fairly recently. From an economical point of view, such applications requires a cheap and efficient material, which is able to convert heat energy into electricity. Magnesium silicide-stannide alloys, also being environmentally friendly and stable at medium temperatures, have a further potential in increasing thermoelectric efficiency by utilizing microstructuring, which has not been much applied to this system. The approach is aimed on reducing the lattice thermal conductivity, while keeping electron transport almost unaffected. In the current work it is suggested to manipulate the microstructure of Mg2Si1-xSnx alloys via formation of endotaxial phases utilizing phase separation. Compositions as well as arrangements of the endotaxial phases are controlled via the heat treatment protocols, which vary mostly depending on the position of the binodal curve, the ratio of Si and Sn in Mg2Si1-xSnx and cooling rate.
Due to the discrepancy in the available quasi-binary phase diagrams Mg2Si-Mg2Sn below the solidus, one focus of the current study was held on the estimation of the binodal curve position. For this, Si-rich Mg2Si1-xSnx alloys of different compositions were treated at different temperatures and their structure was investigated utilizing X-ray diffraction. The study showed an agreement of the results with one of the theoretically calculated binodal curves in the Si-rich region. However, the compositions of the Sn-rich phases did not fit with this or any other known models. Moreover, Sn-rich phases treated at higher temperatures contained less Si, whereas the solubility limit of Si in the Sn-rich phase was expected to grow with temperature, which can be a result of pinning effect provided by particle/grain boundaries.
The acquired approximate position of the binodal curve in the Si-rich region allows one to control the phase separation process, and hence the microstructure. Thus, another focus of the thesis was put on creating the finest and most promising microstructure for thermoelectric materials, i.e. alternating lamellae-type endotaxial phases, which can, in principle, be achieved during spinodal decomposition. Such a microstructure was found during the experiments of the current work. It is shown that when a compound enters the miscibility gap at temperatures that are too low for migration of the atoms over long distances, it rapidly decomposes forming lamellae with similar compositions. Alternatively, if a compound enters the miscibility gap at higher temperatures, higher cooling rates affect the phase separation similarly.
In addition, it is suggested to utilize the Transient plane source technique in quality control and advanced thermal conductivity characterization of complex isotropic and anisotropic materials. Hence, the thesis also includes the recent development of the so-called Structural probe technique, which makes it possible to convert the temperature vs. time function to the unique thermal conductivity vs. probing depth. Since the thermal conductivity is sensitive to the structural constitution of a material, such a function allows one to assess the microstructure variations with depth. The technique was successfully tested on homogeneous and inhomogeneous materials, as well as on materials with macroscopic defects.
Due to the discrepancy in the available quasi-binary phase diagrams Mg2Si-Mg2Sn below the solidus, one focus of the current study was held on the estimation of the binodal curve position. For this, Si-rich Mg2Si1-xSnx alloys of different compositions were treated at different temperatures and their structure was investigated utilizing X-ray diffraction. The study showed an agreement of the results with one of the theoretically calculated binodal curves in the Si-rich region. However, the compositions of the Sn-rich phases did not fit with this or any other known models. Moreover, Sn-rich phases treated at higher temperatures contained less Si, whereas the solubility limit of Si in the Sn-rich phase was expected to grow with temperature, which can be a result of pinning effect provided by particle/grain boundaries.
The acquired approximate position of the binodal curve in the Si-rich region allows one to control the phase separation process, and hence the microstructure. Thus, another focus of the thesis was put on creating the finest and most promising microstructure for thermoelectric materials, i.e. alternating lamellae-type endotaxial phases, which can, in principle, be achieved during spinodal decomposition. Such a microstructure was found during the experiments of the current work. It is shown that when a compound enters the miscibility gap at temperatures that are too low for migration of the atoms over long distances, it rapidly decomposes forming lamellae with similar compositions. Alternatively, if a compound enters the miscibility gap at higher temperatures, higher cooling rates affect the phase separation similarly.
In addition, it is suggested to utilize the Transient plane source technique in quality control and advanced thermal conductivity characterization of complex isotropic and anisotropic materials. Hence, the thesis also includes the recent development of the so-called Structural probe technique, which makes it possible to convert the temperature vs. time function to the unique thermal conductivity vs. probing depth. Since the thermal conductivity is sensitive to the structural constitution of a material, such a function allows one to assess the microstructure variations with depth. The technique was successfully tested on homogeneous and inhomogeneous materials, as well as on materials with macroscopic defects.
multiphase structure
quality control
inhomogeneity tests
Rietveld refinement
Transient Plane Source method
isotropic and anisotropic materials
defects detection
advance measurements of thermal properties
X-ray diffraction
structural characterization
spark plasma sintering
magnesium silicide-stannide alloys
complex behavior
thermoelectric properties
thermal conductivity vs. probing depth
KC-salen, Kemigården 10, Chalmers
Opponent: Eckhard Müller, Institut für Anorganische und Analytische Chemie, Justus-Liebig Universität, Giessen, Germany, and Deutsches Zentrum für Luft- und Raumfahrt, Köln, Germany
Author
Andrey Sizov
Chalmers, Chemistry and Chemical Engineering, Applied Chemistry
Thermal conductivity vs depth profiling using the hot disk technique-Analysis of anisotropic, inhomogeneous structures
Review of Scientific Instruments,;Vol. 94(2023)
Journal article
Influence of Phase Separation and Spinodal Decomposition on Microstructure of Mg₂Si₁₋ₓSnₓ Alloys
Crystal Growth & Design,;Vol. 19(2019)p. 4927-4933
Journal article
A. Sizov, A.E.C. Palmqvist, Effect of thermal treatment on the composition, microstructure and thermoelectric properties of magnesium silicide-stannide
Thermal conductivity versus depth profiling of inhomogeneous materials using the hot disc technique
Review of Scientific Instruments,;Vol. 87(2016)p. 074901-
Journal article
In the current world, we can hardly imagine our lives without energy. We need to power our homes, work places, transport and so on, the list is really long. But natural resources run out, so we need a new ways to generate and save energy. One way, which I work on, uses thermoelectric materials, which are materials that convert heat energy into electricity and vice versa. We won’t need to start fires for heat that thermoelectric materials can convert to electrical energy, we can use waste heat. For example, a car, aircraft, or a huge metallurgical furnace, gives off a lot of heat that is lost to the environment. Instead of letting that heat go to waste, we could apply thermoelectric devices to a heat source and produce electricity. We’re already doing this, just not on our planet. Thermoelectric generators are the only source of energy in a deep space so far; so thanks to them, we are able to study far beyond our solar system.
A wide range of applications for thermoelectric materials, especially their ability to save energy, drives scientists all over the world to look for new materials and ways to make them better. We know that the best thermoelectric materials, ones that are really efficient at energy conversion, have to possess the properties of glass and metal at the same time. They should transfer heat slowly, but be good conductors of electricity. Achieving just the right mix of contradicting properties and even enhancing them requires high tech tools and creative design of thermoelectric materials down to their microstructure.
To construct a certain microstructure, we utilize the phase separation phenomenon. Image an old bottle of jam you’ve had in a cabinet for a long time. You pull out the old bottle and see the jam has sugar crystals floating in it. It happens because jam contains a large portion of dissolved sugar. Over time, sugar gets released (or phase-separates) from jam and clumps together. These sugar clumps attract more and more sugar molecules. So you end up with old jam that has jam with lower sugar content in one phase and another phase which is rich of sugar.
I make thermoelectric materials like the jam separates, except I don't start with a mixture of fruit and sugar. I start with a mixture of metals called an alloy. And we don’t want to wait for years until the alloy gets phase separate. We use so-called phase diagram, which shows us a state of an alloy depending on the temperature and concentration of constituents. For example, did you know that it is possible to dissolve seven glasses of sugar in just one glass of water at 70 degC? But at room temperature, one glass of water can contain only one glass of sugar. So, by reducing the temperature of our sugar saturated water solution from 70 degC to room temperature sugar precipitates. That is exactly how we use the phase diagram to speed up the separation process. By setting a specific temperature, specific treatment time as well as cooling speed we are able to control a shape, size and arrangement of precipitates in the thermoelectric alloys influencing their properties.
By controlling the properties of these materials, I hope to make highly-performing thermoelectric materials that can help us have energy to power our lives
A wide range of applications for thermoelectric materials, especially their ability to save energy, drives scientists all over the world to look for new materials and ways to make them better. We know that the best thermoelectric materials, ones that are really efficient at energy conversion, have to possess the properties of glass and metal at the same time. They should transfer heat slowly, but be good conductors of electricity. Achieving just the right mix of contradicting properties and even enhancing them requires high tech tools and creative design of thermoelectric materials down to their microstructure.
To construct a certain microstructure, we utilize the phase separation phenomenon. Image an old bottle of jam you’ve had in a cabinet for a long time. You pull out the old bottle and see the jam has sugar crystals floating in it. It happens because jam contains a large portion of dissolved sugar. Over time, sugar gets released (or phase-separates) from jam and clumps together. These sugar clumps attract more and more sugar molecules. So you end up with old jam that has jam with lower sugar content in one phase and another phase which is rich of sugar.
I make thermoelectric materials like the jam separates, except I don't start with a mixture of fruit and sugar. I start with a mixture of metals called an alloy. And we don’t want to wait for years until the alloy gets phase separate. We use so-called phase diagram, which shows us a state of an alloy depending on the temperature and concentration of constituents. For example, did you know that it is possible to dissolve seven glasses of sugar in just one glass of water at 70 degC? But at room temperature, one glass of water can contain only one glass of sugar. So, by reducing the temperature of our sugar saturated water solution from 70 degC to room temperature sugar precipitates. That is exactly how we use the phase diagram to speed up the separation process. By setting a specific temperature, specific treatment time as well as cooling speed we are able to control a shape, size and arrangement of precipitates in the thermoelectric alloys influencing their properties.
By controlling the properties of these materials, I hope to make highly-performing thermoelectric materials that can help us have energy to power our lives
Driving Forces
Sustainable development
Areas of Advance
Nanoscience and Nanotechnology
Energy
Materials Science
Subject Categories
Other Materials Engineering
Infrastructure
Chalmers Materials Analysis Laboratory
ISBN
978-91-7905-921-7
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5387
Publisher
Chalmers
KC-salen, Kemigården 10, Chalmers
Opponent: Eckhard Müller, Institut für Anorganische und Analytische Chemie, Justus-Liebig Universität, Giessen, Germany, and Deutsches Zentrum für Luft- und Raumfahrt, Köln, Germany